Television and the like transmitting system



Filed NOV. 19, :1935

(M r fr ,TELEvIlsIpN ANDTHE' LIKE TRANsmTT-ING SYSTEM Aug. 2o, 1940.

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Patented Aug. 20, 1940 UNITED STATES PATENT OFFICE TELEVISION AND THELIKE TRANSMITTING SYSTEM Cecil Oswald Browne, West Acton, London, JohnHardwick, West Drayton,

Frank Blythen,

Application November 19, 1935, Serial No. 50,560 In Great BritainNovember 19, 1934 10 Claims.

The present invention relates to television and the like transmittingsystems.

In most known television systems an object to be transmitted is scannedin contiguous strips and electrical variations are derived from theChanges in brightness of the elemental areas of the object. When theobject has been completely scanned, the operation is repeated. Thiscomplete scanning may take place 24 times per second for example. Sincethe absolute level of brightness has significance to the eye it isnecessary to reproduce in the final picture all changes in brightness ofthe object, however slowly these changes may occur, if the final resultis to be an undis It will be evident that in order'to convey always thecorrect impression of absolute brightness it is necessary to transmitand receive frequencies down to and including zero frequency. Usually,however, the amplifiers serving to amplify the signals prior totransmission have a low frequency ycut-oli, `for example they may becapableof amplifying from `about 10 'cycles per second to 400,000 ormore Cycles per second, and below l0 cycles per second the ampliiicationfalls rapidly.

Moreover some picture signal generators, 4for example certain cathoderay tube transmitters, `do not develop the direct current and very lowfrequency components of the picture signals.

In order to'insert accurately the direct and low frequency components,hereinafter referred -to for shortness as the D. C. component, it isfound to be essential that the amplitude of 'the picture, or combinedpicture and `synchronising signals should attain a fixed maximum orminimum value, relative to the value corresponding to black in thepicture, at suitable intervals `of lower frequency eldl the framescanning iield. The ray is preferably extinguished during the returnstrokes to avoid the .generation of picture signals ,at these times. Y f

rEhe higher frequency field may be called the line scanning field andthe It has been found, however, that spurious unwanted signals arise inthe picture signals during the return strokes of both fields and duringthe scanning strokes at the beginning or end of a line or framejustfollowing the turning on or oi of the cathode ray. Thus, forexample, unwanted signals may occur at the beginning or end of a linescanning stroke or at any time during a line return stroke. Theamplitude of the unwanted signals depends upon the intensity of thescanning ray and also, in a somewhat obscure manner, upon the nature ofthe object being transmitted. Unwanted signals of a similar nature arealso developed under some conditions at the beginnings or ends of theframe scanning strokes or at any time during the frame return` strokes.The cutting oif of the cathode ray during return strokes reduces themagnitude of the disturbing signals 'but itself introduces otherdisturbances. The unwanted signals may consist of excursions in both theblack and white directions and their presence may make it dflicult orimpossible to reinsert direct current and low frequency components intothe picture signals in the manner referred to above.

lt has already been proposed to remove spurious signals, such as thoseabove described, Vby superimposing upon the Vpicture signal intervals inwhich the spurious signalsoccur pulses which drive the signal amplitudebeyond the `peak level of 'the picture signals and by then limiting thesignal amplitude.

It is an object of the present invention, therefore, to provide animproved method and means for partially or wholly removing the spurioussig'- rious signals from aliecting the insertion of the` D. C.component.

According to the present invention, there is provided a method oftelevision transmission inwhich spurious signals occur in the intervalsbetween groups of picture signals and in which the spurious signals Vareremoved, wherein rthe D. C. component of said picture signals isarranged to be present at the point at which the spurious `signals, areremoved, the spurious signals being replaced by a signal having anamplitude rcorresponding to or at least xedly related to picture black.

- The D. C. component ofthe picture signals is presentat the point atwhich the spurious signals `are removed when, at that point, all partsofthe picture signals which correspond to black in the object arerepresented by the same potential.

According to a feature of the present invention there is provided amethod of television transmission in which scanning is eifected with theaid of a cathode ray tube, the cathode ray being periodically deflected,by means of a suitable electrostatic or electromagnetic scanning eld,

from one edge of the area scanned to the opposite edge thereof, thescanning field being then reversed in sense and the ray being deflected,in a return stroke to the first-named edge at a comparatively highspeed, the picture signals so generated being passed to a suitableamplifier, wherein, during the return strokes of the cathode ray, theoutput of the amplier is substantially isolated froin the input thereof,the D. C. component being arranged to be present at the point whereisolation is effected. Thus the amplification of the amplifier may bereduced substantially to Zero during the'return strokes, for example bythe application thereto of suitable pulses which bias one or morecontrol grids thereof to anode current cut-oli. The cathode ray is alsopreferably cut olf 4and in this case the cutting-off and turning-on ofthe ray is advantageously eiected at such timesv that, allowing for anydelay which the picture signals may undergo before reaching the point atwhich isolation is effected, any spurious signals which may arise due tothe cuttingo orr turning-on of the ray arrive at the isolating pointafter the amplification has been reduced, and before the amplication isrestored to its normal level.

According to a further feature of the present inventionl there isprovided a method of television transmission 'in which spurious signalsoccur in the interval between groups of picture signals and in whichthere are superimposed upon the picture signals during said intervalssuppression pulses of such sense and magnitude as to displace thespurious signals beyond the level of picture signals, the spurioussignals being then removed by limiting the amplitude of the picturesignals, wherein it is arranged that the D. C. component of said picturesignals is present at the point at which said superimposition takesplace. To enable the D. C. component to be reinserted if it has'beenlost, the picture signals may be caused, during a part of each linescanned, to assume'a value corresponding to black, these recurrentvalues being utilised for reinsertion purposes. The whole of the signalin the interval between successive lines is preferably, after theelimination of the 'unwanted signals, caused to assume a valuecorresponding to black, for example by superimposing impulses ofsuitable nature upon the signals, and synchionising signals may then besuperimposed upon these black interval signals.

The present invention also provides a method of televisiontransmissionin which the D. C.

' component is inserted with reference to a recurrent, fixed amplitudevalue of the signal and in which there occur in intervals between groupsof picture signals spurious signals which may interfere with theinsertion of the D. C. component, wherein the device by which theinsertion is effected is rendered unresponsive to signals of saidamplitude value during .said intervals.

The invention will now be described, by way of example, with the aid ofthe accompanying diagrammatic drawings, in which Fig. 1 is a schematicdrawing-of a television transmitter,

Fig52 showsthe waveform of picture signals developed by the cathode raytube 3 in the apparatus of Fig. 1 and containing the unwanted signalsreferred to above,

Fig. 3 shows the wave form of synchronising signals before they aremixed with the picture signals,

Fig. 4 shows the wave form of the signals used for cutting off theamplifier and eliminating the unwanted signals,

Fig. 5 shows the wave form of the signals which are used in cutting onthe cathode ray,

Fig. 6 shows the wave form of the picture signals combined with thesuppression signals of Fig. 4, after suitable amplification of thelatter,

Fig. 7 shows the wave form of the combined picture and suppressionsignals after elimination of the unwanted signals,

Fig. 8 shows the interval signals reduced to black level and Fig. 9shows the wave form of the signals of Fig. 8 combined with thesynchonising signals of Fig. 2,

Fig. l() shows apparatus for generating suppression and neutralisingpulses,

Figs. l1 and 13 show detailsof the apparatus of Fig. 10', K

Fig, 12 is an explanatory diagram,

Fig. 14 shows modified apparatus according to the invention,

Fig. 15 illustrates a method of generating neutralising pulses,

Fig. 16 is a circuit-diagram of apparatus for carrying out the presentinvention, and

Figs. 17 to 24 are explanatory drawings illustrating the wave forms ofsignals at various points in the apparatus of Fig. 16.

In all the figures containing graphs, with the exception of Fig. 12a,ordinates represent current or voltage and abscissae represent time.

Referring to Fig. l, picture signals corresponding to an object 2 to betransmitted are generated by means of a cathode ray tube 3, having acathode 4, a modulator electrode 5, two anodes 6 and "I, two pairs ofdeflecting plates 8, 8 and 9, 9 and a screen Il? comprising a mosaic ofphoto-electric elements disposed upon a signal plate. The anodes and l'are held at positive potentials relative to the cathode li by means of asource of current indicated at il, the positive terminal of which isearthed. The mosaic screen is connected to earth through a signalresistance I8.

The deflecting plates 8 and 9 are connected to earth: plate B isconnected to a line-frequency scanning oscillation generator I I, whileplate S is connected to a frame-frequency scanning oscillation generatorI2. Generators II and l2 are fed with synchronising pulses from sourcesI3 and I4, the impulses from which occur at the line and framefrequencies respectively. The form of the impulses generated by sourceI3 is shown in. Fig. 3; source It generates similar rectangularimpulses. The line and frame frequencies are so chosen that the cathoderay scans the mosaic screen III in a series of substantially straightparallel lines, each scanning stroke being followed by a relativelyrapid return stroke.

An'optical system indicated generically at I5 is provided to project animage of the object 2 upon the mosaic screen I0.

The object 2 is provided with a black border indicated at I6, so thatthe cathode ray, after scanning a line of the object 2, scans a blackzone corresponding to the border I6; the border may be provided in anyknown or suitable manner. Means which are further described below, areprovided for cutting on? the cathode ray during the return strokes, andfor turning it on again at the beginnings of the scanning strokes.

A portion of the signal generated by the apparatus described andappearing across resistance I8, is illustrated in Fig. 2, in which thesignal between A and C represents the signals generated in scanning oneline, while the part CD corresponds to a return stroke; AB representspicture signals, BC represents thepart of the signal due to the blackborder l, and 'CD repre.- sents the spurious signals which aregenerated,

during the return stroke, while the ray is cut off.

The portion BC due to the black border constitutes a recurrent signalyof fixed amplitude value which is suitable for use in the re-insertionof the D. C. component of the picture signals. A method by which-thisreinsertion may be accomplished will be described later with referenceto Fig. l, and it will be suilcient to indicate here that the methodinvolves the application of the signal of Fig. 2, the D. C. component ofwhich it is assumed that it is 'desired to rei-insert, to a circuitcomprising a unidirectionally conducting device, and a condenserprovided with a leak resistance, it being arranged that current justflows to the condenser through the unidirectionally conducting devicewhen the part BC of the signal is present, the picture signals ABproducing no ilow of current.

It will be evident that the spurious signals indicated at CD in Fig. 2will interfere with the reinsertion or" the D. C. component by themethod outlined. since the excursions of the signal of Fig. 2 in theupward direction from the straight line ABCD will cause current to flowin the unidirectionally conducting device.

It will be understood that the recurrent xed amplitude value whichserves for reinsertion of the D. C. component need not correspond toblack so long as it is ixedly related to black, and so long as suitablesteps are taken tomake a pre-determined xed current or voltagecorrespond with black in the signal.

It is desired iirstly to eliminate the unwanted signals CD, and secondlyto insert signals suitable for synchronising purposes.

Normally, synchronising pulses derived from the synchronising pulsesources (i3 and i4, Fig. l.) at the transmitter are mixed with thepicture signals in such manner that one line synchronising pulseoccupies between one half and the Whole of the interval between twosuccessive line trains of picture signals, and in order that thesynchronising pulses may be easily separated from the trains of picturesignals at the receiver the synchronising pulses are inserted into thetransmission channel in the blacker-thanblack direction, that is, in theopposite direction from a datum line corresponding to black to thatcorresponding to an increase in brightness of the object. f

It is desirable, therefore, that the signals BCD of Fig. 2 should beconverted into signals of the shape TUXYZ shown in Fig. 9 and this maybe done in the following manner:

Suppression pulses of the kind shown at VILW f Vgreater than thatoccupied by part CD of Fig. 1, that is, the spurious signals.

A set of pulses of the form shown in Fig. 5 is also generated, by meansof source 2E, in xed phase relationshipto the line synchronisingsignais; the pulses of Fig. 5 which last for a time at least equal tothat during which the spurious signals may be developed, will bereferred to as switching pulses, and are applied between the modulator 5and the cathode Il of the tube 3 in such a sense, and at such anamplitude level, that they serve to reduce the intensity of the cathoderay substantially to zero, or, in other words, to cut ofi the ray.

The signals set up across resistance i8, which have the form shown inFig. 2, are fed through picture signal amplier 2|, which ,will beassumed to be an amplifier which is incapable of amplifying the D. C.component of the picture signal to the co-ntrol grid circuit of atetrode valve 22, which has a grid condenser 23 and grid leak 245 in itsgrid circuit, and a biasing resistance 25 shunted by a by-pass condenser2G in its cathode circuit. Suppression pulses from source ltare fed tothe grid circuit of a tetrode valve 2l having a grid condenser 28, gridleak 29, biasing resistance 3l? and by-pass condenser 3l. The anodes ofvalves 22 and 2l are connected through a common anode resistance 32 toythe positive terminal of a source 33 of anode current, the negativeterminal of which is earthed;

the screening grids of these valves are also connected to a suitablepoint in source 33 by connections which, for the sake of simplicity,have been omitted from the drawings.

The picture signals, Fig. 2, are applied to valve 22 in such sense thatan increase picture brightness corresponds to a decrease in the negativepotential of the control grid of that valve, while the pulses of Fig. 4are similarly arranged to drive the control grid of valve 2l lessnegative. The signals of Figs. 2 and 4 are in this way mixed together,the form of the mixed signal, which is set up across resistance 32,being as indicated in Fig. 6, except that the D C. component of thepicture signals is missing, having been lost in amplifier 2l and in thecoupling to valve 22. The amplitude of the suppression pulses is madesuch that the combined amplitude (MNOP, Fig. 6) of suppression pulsesand spurious signals is always brighter than black, and preferablygreater than the maximum amplitude occurring in the picture signals.

As has already been pointed out, the signals set up across resistance 32lack the Di. C. cornponent. This component is now reinserted by means oftetrode Valve Si, which has a grid condenser and a grid leak 36 in itscontrol grid circuit.

The signals applied to valve 34 consist of variations about a datum linesuch that the areas enclosed by the signal envelope above and below thedatum line are equal, and it is arranged that signal excursions in thedownward direction of 6 cause the control grid of valve 32% to becomemore negative. The applied signal causes grid current to ilow in valve3d, and condenser 35 charges up, the control grid potential' thusbecoming more negative. A state is eventually reached in which` only theparts QM? (Fig. 6) of `the signal will cause grid current to ilow, thetime constant of condenser 35 and leak 3S being made such that only aslight change in control grid potential takes place between successiveparts QM?. The datum line is thus changed to one through the peakportions QMP of the signal, and the D. C. componentis thus restored.

The grid base of valve 34, that is, the change in potential required toswing the control grid of valve 3d from zero potential (relatively tothe cathode) to the potential corresponding to anode current cut-off, ismade such that the signal of Fig. 6 is converted in valve lill into asignal of the form shown in Fig. '7; in other words, it is arranged thatparts of the signal of Fig. 6 beyond a line corresponding to the line RSof Fig. '2, lie in the region of ycontrol grid potentials more negativethan that corresponding to anode current out-off, and are thus removed.The valve Sli thus serves not only to re-insert the D. C. component, butalso as an amplitude limiting device.

vThe anode of valve 3d is connected to the positive terminal of source33 through anode resistance 3l, and the signals set up across thisresistance, Whichhave the form indicated in Fig. '7, are fed to thecontrol grid circuit of a further valve 38, which has a grid condenser39, a grid leak Gil, a biasing resistance lll and by-pass condenserr 42.Suppression pulses from source i9 are applied to the control grid of avalve 43 having a grid condenser dll, grid leak 1&5, biasing resistancei6 and by-pass condenser 4l, in such a sense as to drive the controlgrid of valve d3 in the positive sense. The valves i3 and. 34 both havethe same anode resistance 3l, and the outputs of Valves 13 and 3.1i aremixed together across this resistance, it vbeing arranged that thepulses fed to valve i3 are substantially equal in amplitude and oppositein sense to the parts MRS? of the signal oi Fig. 7. The signal :ted tovalve 38 thus has the form shown in Fig.

' 8, the parts MRSP of the signal of Fig. 7 being replaced by intervalsof black, or substantially black level. The pulses of Fig. 4 serve inthe valve 153 as neutralising pulses.

The signal of Fig. 8, which isfree from spurious signals, is now mixedwith synchronising pulses;

the latter are derived from generators i3r and ld, and are applied to avalve d3 having a biasing resistance lid, oy-pass condenser 5, grid leak5l and gr-id condenser 52. Valves 38 and i8 have a common anoderesistance 53; the synchronising pulsesare fed to valve 48 in lsuchsense that they are mixed with the picture signals intheblackerthan-black sense, the mixed signals set up across resistance 53being of the form illustrated in Fig. 9. The mixed signals are then fedto a radio transmitter indicated at 54.

In the arrangement described,` the re--insertion of the D. C. componentis eiected at the point where the superimposition of the suppressionpulses takes place. If desired, the D. C. com ponent may be re-insertedafter the point at which the suppression pulses are superimposed on thepicture signals, and for this purpose, the valve 22 and 2l of Fig. 1 maybe omitted, the picture signals and suppression pulses previously mixedbeing fed directly, in suitable sense, to the control grid circuit ofvalve Bit. It is not necessary that amplitude limitation should beeffected by the re-inserting valve, and the func tions of valve 34 ofFig. 1 may be performed by two D. C. coupled valves, the first effectingreinsertion, and the second, amplitude limitation. It will be seen thatin order to obtain the signal of Fig. 8, in which groups of picturesignals are separated by black, or substantially Vblack intervals,independently of changes in average picture brightness, it is importantthat the D. C. component should be present in the signals fed to .thiscomponent is not re-inserted simultaneously with the super-imposition ofthe suppressing pulses of Fig. 4, it must be re-inserted beforelimitation is eiiected. When the D. C. component is present at thelimiting valve, the part MRSP of the signal of Fig. 7 is the same inevery interval, and a neutralising pulse of constant amplitude can thusbe employed. The pulses of Fig. 4 can be used not only as suppressionand neutralising pulses, but also to reduce the amplification of one ormore of the valves of the picture signal amplifier 2l, preferablysubstantially to Zero, in this way isolat-. ing the output of theamplifier from the input thereof. For this purpose, the pulses of Fig. 4may be fed over lead 55 from source i9 to ampliiier 2l, and applied tobias the grids of the amplifier valves to or beyond anode currentcut-off. Preferably at least the valve nearest the source of picturesignals is out 01T in this way and other later valves may also be outoff if desired.

It will be noted that owing to the shapes of the pulses of Figs. 4 and5, the amplifier 2l is out off at rV (Fig. 4) before the cathode ray is`cut off at I (Fig. 5) and the ray is again turned on at L (Fig. 5)before the amplifier is turned on at W (Fig. e). In determining the`shapes of the pulses of Figs. 4 and 5, due regard must be had toanyidelays which the picture signals may undergo up tothe point wherethe amplifier 2l is cut off, so that the spurious signals always arriveat that point after the amplifier is out ofi and cease before it isturned on again. spurious signals arising from the cutting oland'turning on of the cathode ray are in this way at least substantiallysuppressed.

Instead of producing a black signal BC (Fig. 2) at the end of each line,a whiter-than-white signal may be produced. By a whiter-than-whitesignal is meant a signal value in the white direction from black andfurther removed from black than full White in the image of the object.Such a signal may be producedfor example by forming an image of anelongated source of light upon one side of the image upon the mosaicscreen, and for this purpose, the black border i6 of Fig. 1 is arrangedto be replaced by this bright image. The brightness oi the marginalimage is made such that it will always be greater than that of thebrightest part of the image of the object 2 formed upon the screen iii.The D. C. component of the picture signals is .re-inserted withreference to the recurrent whiterthan-white,l signal value instead ofblack value QM in Fig. 6.

Suppression pulses from source i9 are arranged, as before, to drive theunwanted signals so far in the white direction that they can be removedby an amplitude limitation, in a manner such as has already beendescribed. The resultant interval signal, which then has awhiter-than-white value, is brought to the condition TUXYZ, shown inFig. 9, by superimposing upon the picture signals synchronising pulsesof the form shown in Fig. 2, these pulses being derived from sources i3and lil and being superimposed upon the picture signals in such a sense,and at such an amplitude level that signals of the form of Fig. 9 areobtained. In this way, the whiter-than-whte pulses are removed from thesignals as well as the spurious signals. If it is found that thespurious signals extend beyond the whiter-than-white signals in such away that they would interfere with the re-insertion of the D. C.component, pulses of the kind shown in Fig. i may be superimposed uponthe signals at the reinsertion point, these signals being derived fromsource i9 and being arranged tc be in such sense as to drive thespurious signals to a sufficient extent in the black direction. Theprocedure thereafter may be as before.

The pulses of Figs. 4 and 5 may be generated by means of a disc havingtwo series of apertures of suitable shape arranged on concentriccircles, the disc rotating between a pair of light sources and twoco-operating photo-sensitive cells. Apparatus of this kind will bedescribed later with reference to Figs. l and l1. The synchronisingsignals of Fig. 2 may be generated by a second series of apertures inthe same disc, arranged to co-operate with a separate cell.

In a further arrangement, the D. C. component is not re-inserted withreference to a black or whiter-than-white signal arranged to occur atthe end of each line, in the manner described above, but is re-insertedwith reference to peaks of the picture signal in the black direction.'I'he black border or bright image i6 referred to above may then beomitted, and it should be noted that in this further arrangement, thedurations of the pulses of Figs. 2, 4- and 5 must be chosen with thefact in mind that the interval BC of Fig. 2 is no longer present.

It has been found diiiicult to ensure that the pulses employed asneutralising pulses have exactly the same duration and shape as thesuppression pulses. If they have a diierent duration, narrow, peakypulses in one sense or the other will remain after the superimpositionof the neutralising pulses. If these residual pulses are in the whitedirection, they may cause white lines to appear at the edge of the finalpicture. If the residual pulses are in the blacker-thanblack directionthey are likely to disturb the synchronisation, since the synchronisingsignals are usually in this direction in the transmitted signal.

It may therefore be arranged that the durations of the suppression andneutralising pulses are different and such that the residual pulses arein the blacker-than-black direction. The residual pulses are thenremoved by means of a suitable limiting device before the admixture ofthe synchronising pulses. Usually the suppression pulses are applied inthe white direction and they are then made of shorter duration than theneutralising pulses.

One method of generating suppression and neutralising pulses ofdifferent duration will now be described with reference to Figs. 10 to13 of the accompanying drawings. Referring to Fig. 10, a disc 56 isdriven, by means of a motor 5l, to rotate between a light source 53 andan apertured diaphragm 59, behind which is a photo-electric cell G9. Thepulses set up in the cell 69 are fed to an apparatus Si, which comprisesan amplier and means, which will be described with reference to Fig. 13,for dividing these pulses into two separate sets. Fig. 11 is a frontelevation of the disc 56, which has a plurality of substantiallyrectangular apertures G2 formed therein.

Referring to Fig. 12, the graph a represents the intensity of the lightfalling on the diaphragm 59 plotted against time as abscissae, while brepresents the form of the aperture in the diaphragm 59. The curve crepresents the form of the pulses set up in cell 69, amplitude of pulsebeing plotted against time as abscissae. Each aperture B2 in disc 5Sgives rise to a tapered pulse ABCDEF of the form shown in Fig. 12a, andthese pulses, after amplification in amplifier 6I, are fed to thecontrol grid circuit of a valve 63 through a grid condenser 61% withwhich is associated leak resistance 65. d

The valve S3 is biased by means of biasing resistance G9 and condenserlil to a point which is more negative than that corresponding to anodecurrent cut-off, and the pulses of Fig. 12o are arranged to drive thecontrol grid of valve 63 less negative. vThe value of the grid bias isso chosen that only the upper portions BCDE of the pulses cause anodecurrent to now; the anode of valve 63 is connected through resistance 68to the positive terminal of a source 'I9 of anode current, and pulses ofthe forml BCDE appear at terminal 69, and are employed as suppressionpulses.

'Ihe pulses ci Fig. 12o are also applied to a valve li, having a gridcondenser l2, leak I3 and anode resistance lli, which is so biased bymeans of resistance 'l and condenser 'i6 that it effects no amplitudelimitation of the pulses, but reverses them in sense and applies them tovalve 'Vl by means of condenser 1S and leak resistance 19. Valve l1 isso biased by meansl of resistance 80 and condenser 8l that anode currentonly flows Vtherein for the parts ABEF of the pulses of Fig.

12e. The anode or valve il is connected to the positive terminal ofsource 'i9 through resistance 82, and pulses of the form ABEF appear atthe terminal 83, whence they are led away to serve as neutralisingpulses.

The neutralising pulses, which are constituted by the lower portions ofthe tapering pulses of Fig. 12e, are thus arranged to be of longerduration than the suppression pulses, which are constituted by the upperportions. The suppression pulses are conveniently mixed with the picturesignals in the white sense, so that when, after the reinsertion of theD. C. component, and subsequent limiting, the neutralising pulses aresuperimposed, the residual impulses are in the blackerthan-black sense,and can be removed by an amplitude limitation by a method such as hasalready been fully described.

Instead of generating the suppression and neutralising pulses as abovedescribed from a single set of tapering pulses, the two sets of pulsesmay be generated separately. For example, two sets of pulses may begenerated optically in a manner such as that described with reference toFigs. and 11, with the aid of two sets of apertures of different widths.Alternatively, the pulses may be derived from two separate electricalpulse generators.

It is however, advantageous to use what is effectively a single set ofpulses, namely the two parts of the tapering pulses, for bothsuppression and neutralising, becausea change in the tapering pulse, forexample in its length, affects both suppression and neutralisationequally.

A modified circuit arrangement, applicable whether the suppression andneutralising pulses are of equal or unequal durations will now bedescribed with reference to Fig. 14. Referring to Fig. lli, the signalsci the form of Fig. 6, containing their D. C. component, are applied tothe inner control` grid of a hexode valve 84 through grid condenser andleak 86, the neutralising pulses being applied to the outer control gridthrough grid condenser 8l, with which is associated leak 98 and a source89 of biasing potential. The inner grid is biased by means of source 90.The picture signals are applied in such sense that they render the innergrid less negative, whilst the neutralising pulses applied to the outergrid render it more negative. A resistance 9| providing negativereaction is arranged in the cathode lead of the valve 84 to straightenthe characteristic and sharpen the bottom bend. The anode of valve 84 isconnected to the positive terminal of a source 92 of anode currentthrough a resistance 93, and the screening grids are connected to apoint at a positive potential in source 92. It is arranged that, withthe outer control grid at the potential it assumes in the intervalsbetween neutralising'pulses, picture black corresponds to anode currentcut-oi; signals of the form of Fig. 8 can thus be derived from terminal94.

Where the neutralising pulses are of greater duration than thesuppression pulses, the residualpulses are in the negative sense beyondthe point corresponding to anode current cut-01T in valve 84, andproduce no change of anode current.

Since the amplitude (with respect to black) of the residual pulsesbefore they are removed is dependent solely upon the amplitude of theneutralising pulses, which is kept constant, the D. C.

component may be reinserted with reference to the peaks of theseresidual pulses before the latter are removed. It may be desirable to dothis, for example, when the D. C. has been Wholly or partially lostbetween the point at which it is rst inserted and the point at which theresidual pulses are removed.

Where these residual pulses are applied in the negative sense to thegrid of a valve, the D. C. may be re-inserted with the aid of a dioderectifier in parallel with the grid circuit of the valve as described inBritishPatent specication No. 422,906. l

In an alternative method of generating suppression and neutralisingpulses of different durations, suppression pulses are generated in anyconvenient manner, and neutralising pulses are generated from thesuppression pulses with the aid of a hexode valve. Referring to Fig. 15,the suppression pulses are applied directly and in the negative sensefrom terminals 99 to the outer control grid of a heXode valve 95 andthrough a delay network 96 which comprises inductances 91 and shuntcondensers 98, and also in a negative sense, to the innercontrol grid.The outer control grid has a condenser |08 and a leak Il, while theinner grid has a condenser |02 and a leak |03. The anode is connectedthrough resistance |04 to a source |05 of anode current, to apoint inwhich the screening grids are also connected. It is arranged that thedirectly applied pulses cause the anode current of the hexode to fall tozero and that the pulses arriving through the delay network serve tohold the anode current at zero. The resulting pulses at the terminal |06are therefore of longer duration than the suppression pulses. Thesuppression pulses, which are to be superimposed upon the picturesignals, may be subjected to half the delay to whichV the signalsapplied tothe inner grid of the hexode are subjected, so that the centrelines of the suppression and neutralising pulses are substantiallycoincident.

'I The methods above described for eliminating spurious signals from theline intervals may also be applied to remove spurious signals from theframe intervals. vThe frame intervals are usually longer than the line.intervals and the suppression and neutralising pulses are then made ofsuitably longer duration.

A further example of the present invention will now be described withreference to Figs. 16 to 24.

In Fig. 17 is shown a part of a picture signal. In this iigure, as inFigures 19, 21, 22 and 24, the sense corresponding to an increase inbrightness is indicatedV by an arrow W. In Fig. 17 the periods P areperiods of scanning complete lines of the object and the periods P1 areperiods of scanning half lines of the object, the signals being thosedeveloped in interlaced scanning where the object is completely scannedin two traversals thereof, the lines traced out in one traversalinterlacing with those traced out in the next traversal. The periods LIare the line intervals and the periods FI are the frame intervals.During these intervals spurious signals S may occur.

Ihe signal of Fig. 17 does not contain the D. C. component and the Zeroline o therefore is such that the areas enclosed by the wave form aboveand below it are equal to one another.

The signal of Fig. 17 is applied to the grid of a valve |07 in Fig. 16in such sense that signals in the direction W make the grid morenegative. The valve |01 has a resistance |08 in its cathode circuit, andis arranged in such a way that the oscillations derived from the cathodesubstantially correspond to those applied to the grid. The anode ofvalve |07 is connected to the positive terminal of a source |89 of anodecurrent. Oscillations from the cathode of valve |01 are applied to thecontrol grid of a valve ||0 through a condenser A tetrode valve ||2 isconnected as shown between the grid and cathode of the valve H0, aresistance ||3 being connected between the anode and cathode of thevalve. Ihe cathode potential of valve ||2 is adjusted so that valve I8is biased to a desired point on its characteristic, and for this purposeis connected to earth through a bias resistor ||4 shunted by a condenser||5.

The anode and cathode of the tetrode |2 acts as a diode, and currentflows through the diode on the positive peaks of the signal (that is thepeaks in the black direction) thereby charging condenser l|| and makingthe grid of valve ||9 more negative. and resistance E3 is made longcompared with the intervals P, (Fig. 17). When the steady state isreached, current will flow through valve ||2 only on the extreme tips ofthe signals and D. C. will be established on the grid of valve H0. Itwill, however, be clear that parts of the spurious signals S may projectbeyond black into the blacker-than-black region and interfere with theinsertion of D. C. by the device H2, ||3. To prevent this, negativepulses such as are shown in Fig. 18 are applied'to the control grid ofthe valve ||2 from a source H6. These pulses serve to stop current flowin the anode-cathode path of the valve H2 during the intervals LI and FIof Fig. 17 and so render the re-inserting device unresponsive toblacker-than-black peaks which may occur in these intervals. There-insertion of D. C. is thus unaffected by the spurious signals andtakes place with reference to the blackest parts of the picture signalsin the intervals P and P1. The resulting signal at the grid @of valve||9 is shown in Fig. 19, the Zero axis O corresponding to picture black.

The anode oi valve Il@ is connected to the anode of ay tetrode valveandthrough resistance Yl I9 and a condenser ||8 in parallel to The timeconstant of condenserV the grid of a valve |20. The anodes of valves ||0and lil are connected to the positive terminal of source its throughresistances |2| and |22, a decoupling condenser |23 being also provided.A resistance lili is connected between the grid of valve 82d and earth,and the anode of this valve is connected to the positive terminal ofsource idg.

To the control grid of valve is applied a signal which is of the formshown in Fig; 20 and is derived from source H6. The signal at the commonanodes of valves HQ and H7 will thus be as shown in Fig. 2l, thespurious signals S being displaced wholly beyond black level representedlby the axis O in the blacker-than-black direction.

These signals are conveyed to the grid of valve l2?) through couplingmeans, including elements il, E22, Sie, 52d, lit? and |23, which serveto pass all frequency components of the signal, including zero frequencyor D. C. The valve iZO is so'adjusted that anode current ceases to flowat a signal -level corresponding toblack, so that at the catho-de ofthis valve (which is connected to earth through resistance E25) thesignal voltage has the forni shown in Fig. 22, the axis O correspondingto black. This signal is applied to the grid of a valve the cathodeofwhich is connected to the cathode of another valve |2`| and thence toearth through resistance t28. To the grid of valve l2? are appliedsynchronising signals of the form shown in Fig. 23 and the signal outputfrom the cathodes of valves |26 and |21 will then have the form shown incurve 2d. The signals of 23 are derivedy from source |29.

It will be seen that the spurious signals have been removed and the D.C. component has been inserted.

We claim:

l. In a television transmission system in which video signals having adirect current component and spurious signals are developed and in whichthe direct current component is suppressed in the passage of the videosignals through the transmission apparatus, the method of re-insertingthe direct current component exclusive of the spurious signals whichcomprises the steps of developing suppression waves, combining thesuppression waves with the spurious signals to raise yso the amplitudelevel thereof, limiting the value of said combined waves to a valueoutside the range of the spurious signals, and re-inserting the directcurrent component at a portion of the system through which passes onlythe limited combined wave substantially simultaneously with the limitingthereof.

2. The method in accordance with claim 1 wherein there is provided theadditional steps of developing neutralizing Waves and combining theneutralizing waves with the limited Waves in such as to removesubstantially the limited waves.

3. In a television system wherein there is included means incapable oftransmitting direct current low frequency alternating current, means fordeveloping trains of video signals having a'direct current componentfrom anobject to be scanned, and in which there are included spurioussignals, means for developing a signal xedly related to a xed shade inthe object to be scanned, means for developing suppression pulses, meansfor combining said suppression pulses with said spurious signals wherebythe' level of said spurious signals is changed, and means for limitingthe value of the combined suppression pulses and spurious signals passedto the system to a value outside the range of said spurious signals'saidlatter means also acting to re-insert rthe direct current component ofthe video signal with respect to the signal xedly related to a fixedshade in the object to be canned. 4. Apparatus in accordance with claim3 wherein said means for limiting the amplitude ci the combinedsuppression pulse and spurious signal and also acting to re-insert thedirect current component of the video signals comprises a thermionictube having the input circuit thereof of such parameters as to block thecurrent now in the tube at an amplitude short of the spurious signallevel and to maintain a substantially fixed ing to the direct currentcomponent.

5. Apparatus in accordancey with claim 3 wherein there is provided inaddition means for cancelling the limited pulses said means comprisingmeans for developing neutralizing pulses, and means for combining saidneutralizing pulses with said limited pulses in such phase as tosubstantially cancel said limited pulses.

6. In a television system wherein there is included means incapable oftrans itting direct current and low frequency alternating current, meansfor developing trains of, video signais having a direct currentcomponent from an object to be scanned and in which there are includedspurious signals, means for developing a signal xedly related to pictureblack in the object to be scanned, means for developing suppressionpulses, a thermionic vacuum tube having anode, cathode and atleast onecontrol electrode, means for combining said suppression pulses and saidspurious signals in such phase as to increase the amplitude level ofsaid spurious signals, means for impressing said combined spurioussignals and suppression pulses onto the control electrodecathode circuitof. said thermionic tube, means for impressing said video signals andsaid signal xedly related to picture black onto the controlelectrode-cathode circuit of said tube, a time constant circuitconnected in the control electrode-cathode circuit 'of said tube havingparameters of such value that said tube is blocked for wave amplitudesof the range of the spurious signal amplitude, and said controlelectrode is maintainedat a substantially constant bias for wavescorresponding to the signal Xedly related to picture black, whereby -thespurious signals are removed, and the direct current component of thevideosignals is re-insertedby the same thermionic tube.

7. Apparatus in accordance with claim 6 wherein there is provided inaddition means for cancellingthe limited pulses said means comprisingmeans for developing neutralizing pulses, and means for combining saidneutralizing pulses with said limited pulses in such phaseras tosubstantially cancel said limited pulses.

8-. In a television system wherein there is included means incapable oftransmitting direct current and low frequency alternating current, meansfor developing trains of. video signals having a direct currentcomponent from an object to be scanned and in'Which there are includedspurious signals, means for developing a signal rlXedly related topicture black in the object to be scanned, means for developingsuppression pulses, a thermionic vacuum. tube having anode, cathode andat least one control electrode, means for combining said suppressionpulses and said bias on said input circuit of a value correspondspurioussignals in such phase as to increase the amplitude of said spurioussignals, means for impressing said combinedspurious signals andsuppression pulses ontothe control electrodecathode circuit of. saidthermionic tube, means for impressing said video signals and said signalXedly related to picture black onto the control electrode-cathodecircuit of said tube, a time constant circuit connected in the control`electrodecathode circuit of said tube having parameters of such valuethat said tube is blocked for wave amplitudes 0f the range of thespurious signal amplitude, said control electrode is maintained at asubstantially constant bias for waves corresponding to the signal xedlyrelated toy picture black, whereby the spurious signals are removed, andthe direct current component of the video signals` is re-inserted by thesame thermionic tube, means for developing neutralizing pulses, meansfor combining said neutralizing pulses with the limited combinedsuppression pulses and spurious signals in such phase as tosubstantially cancel the limited combined suppression pulses andspurious signals, means for developing synchronizing signals in theintervals between trains of picture signals, and means for insertingsaid synchronizing signals during at least a portion of the intervalduring which the cancelled ysuppression pulses have been present.

9. Apparatus in accordance with claim 6 wherein there is provided inaddition means for cancelling the limited pulses said means comprisingmeans for developing neutralizing pulses and means for combining saidneutralizing pulses with said limited pulses substantially 180 .degreesout of phase therewith, the neutralizing pulses and the suppressio-npulses having different durations.

10. Television transmitting apparatus comprising picture signalgenerating means of such nature that spurious signals may be generatedin intervals between groups of picture signals, means for generatingsuppression pulses of predetermined suitable amplitude, means or mixingsaid pulses with said picture signals and spurious signals in thepresence of the D. C. component of said picture signals, a heXode valve,means for feeding superimposed picture signals, spurious signals andsuppression pulses to one control grid of said Valve in such sense thatan lincrease in picture brightness is represented by a positive signal,means for generating neutralising pulses of predetermined suitableamplitude, means for feeding said neutralising pulses to the othercontrol grid of, said valve in such sense as to drive the potential ofthat grid in the negative sense, and means for ensuring that, inintervals between successive neutralising pulses, a picture signalrepresentative of black causes the flow of anode current substantiallyto cease.

CECIL OSWALD BROWNE.

JOI-IN HARDWICK.

FRANK BLYTI-IEN.

ERIC LAWRENCE CASLING WHITE.

